Membrane concentrator

ABSTRACT

Apparatus for concentrating a nebulant comprising a nebulant flow conduit and a counter-flow conduit, or preferably, a plurality of alternating nebulant flow conduits and corresponding counter-flow conduits eg in layered or coaxial arrangement. and wherein at least a portion of the nebulant flow conduit and said counter-flow conduits define respective opposed sides of a gas permeable membrane. In use a nebuliser is in communication with the nebulant flow conduit and the nebulant flow and counter-flow are in the same or opposite directions and act to concentrate the amount of active in a droplet eg from 35 wt % to 60 wt % hydrogen peroxide in water to disinfect and/or sterilize an article.

FIELD OF THE INVENTION

This invention relates to a method and apparatus for concentratingaerosols, such as may be used for example in disinfecting or sterilizinga surface. The method and apparatus are particularly suited fordisinfecting or sterilizing medical instruments but are not limited tothat use.

BACKGROUND OF THE INVENTION

The present application incorporates by reference the whole of theapplicants co-pending applications AU2005904181, AU 2005904196 and AU2005904198.

As outlined in these co-pending applications, sterilization processesand apparatus that address the following criteria are highly desirable:

(a) avoid the need for vacuum

(b) avoid the need for a rinsing step

(c) avoid the need for temperatures above 60° C.

Many of the process of the prior art employ vacuum and or rinsing steps.These have the effect of increasing the complexity and cost of theapparatus required, and can lengthen the time of the disinfection orsterilization process considerably (meaning more downtime for expensivemedical instruments). The use of high temperatures can also increase thecomplexity and cost of sterilization instruments, but more importantly,it can damage many materials.

It is desired to provide disinfecting methods and apparatus that meetthese criteria, while achieving the highest possible efficacy inpathogen destruction, especially when treating occluded, mated and lumensurfaces.

It is desirable that the disinfecting methods use hydrogen peroxide.Hydrogen peroxide at low concentrations is safe to transport, sell andhandle and is extremely well known, with little or no regulatorybarriers to its use. However, there are problems with those methodswhich require high concentration hydrogen peroxide as a startingmaterial. For example, commercial vapour and plasma processes use as astarting material corrosive and irritating 60% peroxide solutions whichrequiring special packaging and handling precautions.

When hydrogen peroxide is used in the form of small droplets (sprayed,ultrasonically nebulized, etc), the particles have a tendency to depositas droplets on surfaces and the residual layer of peroxide is apotential problem. Medical instruments, food packaging and otherdisinfected items need to be stored dry to avoid re-contamination.Importantly, surgical instruments must not contain residual peroxide atlevels higher than 1 microgram/ sq. cm.

However, eliminating residual peroxide is very difficult. It requireseither washing which introduces the associated problems previouslydiscussed in our copending applications in connection with liquidsystems, prolonged periods of high temperature drying (which completelynegate any advantages arising from fast kill times and low processtemperature) or requires use of catalase or other chemical means todecompose peroxide (which still requires drying and which creates aseries of problems with the residual chemicals left on instruments) orthe use of vacuum. Accordingly, it is desirable to provide a system thatuses the minimum possible amount of peroxide to achieve a desiredeffect.

Any discussion of the prior art throughout the specification should inno way be considered as an admission that such prior art is widely knownor forms part of common general knowledge in the field.

OBJECTS OF THE INVENTION

It is an object of the invention to provide improved methods andapparatus for disinfecting or sterilizing medical instruments whichavoids or ameliorates at least some of the disadvantages of the priorart.

It is an object of preferred embodiments of the invention to provideimproved methods an apparatus capable of concentrating and improving theproperties of an aerosol.

Unless the context clearly requires otherwise, throughout thedescription and the claims, the words ‘comprise’, ‘comprising’, and thelike are to be construed in an inclusive sense as opposed to anexclusive or exhaustive sense; that is to say, in the sense of“including, but not limited to”.

BRIEF STATEMENT OF INVENTION

According to a first aspect, the present invention provides apparatusfor concentrating a nebulant comprising:

a nebulant flow conduit;

a counter-flow conduit; and

wherein at least a portion of said nebulant flow conduit and saidcounter-flow conduit define respective opposed sides of a gas permeablemembrane.

According to a second aspect, the present invention provides apparatusfor concentrating a nebulant comprising:

a plurality of alternating nebulant flow conduits and correspondingcounter-flow conduits; and

wherein at least a portion of said each nebulant flow conduit and anadjacent counter-flow conduit define respective opposed sides of a gaspermeable membrane.

The alternating nebulant flow conduits and counter-flow conduits may bein a layered configuration. Alternatively, they maybe in a concentric,coaxial tubular arrangement.

Each nebulant flow conduit comprises an inlet and an outlet. Eachcounter-flow conduit comprises an inlet and an outlet. Preferably, thenebulant flow and counterflow are in opposite directions. However, theymay in the same direction, or any other direction, eg perpendicularflows.

According to a third aspect the invention provides apparatus forconcentrating a nebulant comprising:

a nebulant flow conduit;

at least two counter-flow conduits; and

wherein at least a portion of said nebulant flow conduit and saidcounter-flow conduits define respective opposed sides of gas permeablemembranes.

According to a fourth aspect the invention provides apparatus forconcentrating a nebulant comprising:

at least two nebulant flow conduits;

a counter-flow conduit; and

wherein at least a portion of said counter-flow conduit and saidnebulant flow conduits define respective opposed sides of gas permeablemembranes.

According to a fifth aspect the present invention provides a method forconcentrating a nebulant comprising the steps of

(1) providing a nebulant flow of an active in a solvent and having afirst active:solvent ratio to a first side of a gas permeable membrane;and

(2) providing a counter-flow of a gas to a second side of thegas-permeable membrane whereby to increase said ratio on the first sideto a second active:solvent ratio greater than the first active:solventratio.

The concentrated nebulant is preferably used to disinfect and/orsterilize an article.

The nebulant is preferably a nebulant of water and a biocide. Mostpreferably, the biocide is hydrogen peroxide. The first active tosolvent ratio is preferably about 30 wt %.

The second active:solvent ratio is preferably about 70 wt %. Thecounter-flow of gas is provided at a rate and for a time such that thesecond ratio is not capable of further increase.

For preference the gas is air, more preferably humidity conditioned air.

The semi permeable fabric or membrane may be a woven, or non-wovenfabric, or it may be a sheet or film or a combination thereof and may beof a single layer or multilayer construction.

The term “semi permeable membrane” is used herein where the contextpermits to include all such fabrics and membranes having the selectedproperties. The semi permeable membrane may be hydrophobic orhydrophilic in nature.

The semi permeable membrane is selected to ensure that nebulantparticles are initially unable to permeate.

In this specification where the context permits references to a semipermeable fabric or membrane include fabrics or membranes suitable forpervaporation as well those only suitable for simple permeation, andreferences to permeation include references to pervaporation. Othermembranes than those described and membranes may be used and may includemembranes suitable for pervaporation.

In a highly preferred embodiment a peroxide solution having an initialconcentration of at least 6%, preferably 20%-35%, and more preferably30%-35%, is nebulized. Preferably the solution is nebulized in anultrasonic nebulizer operated at 2.4 MHz which generates an aerosol inwhich particles having a size range distribution of about 1-10 micronsare suspended in an air stream. As herein used the term “nebulant”describes droplets of liquid (i.e. finely divided liquid particles)entrained in a gas stream. A system of liquid droplets entrained orsuspended in a gas is an “aerosol”.

Without wishing to be bound by theory, it is believed that as watervapour permeates through the membrane, water evaporates from thenebulant droplets in order to restore the equilibrium vapour pressurewithin the nebulant flow conduit. Continuing evaporation from thedroplets results in the peroxide solution in the nebulant becoming moreconcentrated, and in the droplets shrinking in size.

These smaller more concentrated nebulant particles are significantlymore effective as a sterilant than prior art hydrogen peroxide vapourpossibly because a much higher concentration of sterilant is obtainableper unit volume than with vapour and is more effective than prior artperoxide nebulant sterilants and processes.

Air permeating into the nebulant flow conduit is sterile by virtue thatthe membrane is not penetrable by micro-organisms.

According to a sixth aspect the invention provides a process accordingto any one of the preceding aspects wherein the semi permeable membraneis selected to remove one or more vapours by a process of pervaporation.

Although the invention is herein described with reference to hydrogenperoxide as the biocide, the invention is equally applicable when thebiocide was another peroxide or peroxy compound, or could be used withother known vaporizable biocides or biocides when dissolved in suitablesolvents (which need not be aqueous). Furthermore, although it is highlypreferred to introduce the biocide as an aerosol, in less preferredembodiments the biocide can be introduced as a vapour and the vapoursubsequently removed at atmospheric pressure by an exterior current ofair (or other fluid) adjacent the membrane exterior. Introduction of thebiocide as an aerosol is greatly preferred because much higher initialdensities of biocide per litre of container can be achieved than with avapour. Our co-pending application indicates that aerosols according tothat invention, which are believed to be the same as or similar to theaerosols produced in this process are more effective than vapour.

According to a seventh aspect the present invention provides a methodfor disinfecting or sterilizing an article or article part comprisingthe steps of

(1) enclosing the article or article part inside a first containerhaving a wall of which at least a part is a semi permeable fabric ormembrane;

(2) the semi permeable fabric or membrane being selected to allow vapourto pass from inside to outside of the container while providing abarrier against entry of micro-organisms and against exit of nebulantparticles;

(3) admitting a biocide solution comprising a biocide dissolved in asolvent to a second container;

(4) concentrating the biocide in the second container by removal ofsolvent, to form a concentrated biocide

(5) introducing the concentrated biocide as a liquid or a vapour or acombination thereof from the second container to the first; and

wherein steps (4)-(5) are conducted at or above atmospheric pressure.

In preferred embodiments according to the sixth aspect a hydrogenperoxide solution in water of for example 35% concentration is firstlyconcentrated as a nebulant in one chamber by removal of water through amembrane at atmospheric pressure. The concentrated nebulant is thenadmitted to another chamber which is desirably a bag or other containerhaving a semi permeable membrane as defined as a wall or part thereofwhich is then sealed. This allows the article to be sterilized andstored sterile in the second container and permits removal of residualhydrogen peroxide and water. Preferably the invention provides inparticular, a nanonebulant having 90% of particles in the 3-5 μm rangeand a peroxide concentration of >70 wt % and a water concentration ofless than 30 wt %.

According to an eighth aspect the invention consists in a nano-nebulantcomprising a solution of hydrogen peroxide suspended in finely dividedform wherein the liquid particles have concentration of greater than 60wt % of hydrogen peroxide, and an average diameter of less than 1.0micron. Preferably the droplets have an average diameter of less than0.8 microns.

It will be appreciated that in prior art aerosol systems the peroxideliquid particles have had a concentration of less than 35% wt ofhydrogen peroxide and an average diameter in excess of 2 microns. Therelationship between particle size and fall velocity of particles in anaerosol is non linear, and so a small reduction in particle diametergreatly increases suspension stability as well as increasing the totalsurface area of the gas/liquid interface.

Desirably, the nebulant according to the seventh aspect has a peroxidedensity (grams of hydrogen peroxide/litre of aerosol) much greater thanthe peroxide density of a vapour at just below its saturation limit at acorresponding temperature and humidity.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a reproduction of a figure from U.S. Pat. No. 4,797,255 whichshows (curve A) how the boiling point of a water/peroxide mixturechanges with concentration at atmospheric pressure and (curve B) how thegas composition changes.

FIG. 2 is diagram of a first simple embodiment of the present invention.

FIG. 3 is a diagram of a sterilizing apparatus showing thepre-concentrator of the present invention

FIG. 4 is a more detailed schematic diagram of a sterilizing apparatusshowing the pre-concentrator of the present invention

FIG. 5 shows a further embodiment of the present invention.

FIG. 6 shows flow patterns of nebulant and counter flow in an embodimentof the present invention

FIG. 7 shows the plates that may be used to separate semi permeablemembranes in those embodiments of the present invention that use stackedarrays.

FIG. 8 shows results from a membrane concentrator of the presentinvention.

FIG. 9 shows an ultrasonic probe in disinfecting arrangement with anebuliser of the present invention.

DESCRIPTION OF PREFERRED EMBODIMENTS

The invention will now be described in the context of sterilization, butit will be appreciated that the pre-concentrators and pre-concentrationmethods of the present invention can be used in a variety of fieldswhere concentrated nebulants are desired, eg drug delivery,painting/printing, food preparation, materials fabrication and the like.For example, a number of such processes have been described (U.S. Pat.No. 6,451,254, U.S. Pat. No. 6,673,313 and U.S. Pat. No. 6,656,426) allof which require involve concentrating a hydrogen peroxide solution bylowering the pressure to preferentially evaporate water and removing thewater through a vacuum pump prior to vaporising the solution.

The general pre-concentration process of the present takes place in thecontext of the following, and can be seen with reference to FIG. 3. Anarticle to be sterilized 1 is placed into a sterilization chamber 2. Thesterilization chamber 2 may be any suitable container, butadvantageously is a bag made from a semi-permeable membrane, or a sealedcontainer having a window of a semi-permeable membrane 3.

A pre-concentrator chamber of the present invention 4 is connectedupstream of the sterilization chamber 2. The sterilization chamber 2 andpre concentrator 4 are connected such that flow between thepre-concentrator and sterilizing chamber can be opened or closed by wayof a valve 5.

An ultrasonic nebulizer 6 is connected upstream of the pre-concentratorchamber. A hydrogen peroxide solution having a starting concentrationpreferably of around 30-35% is nebulized in an ultrasonic nebulizer.

The nebulizer 6 may be fed with sterilant solution on a continuous orintermittent basis from a bulk supply 7, e.g. while maintaining apredetermined liquid level in the nebulizer, or may be provided with asingle shot dosing system for example a cartridge providing sufficientsolution for one or a plurality of sterilization cycles. Alternatively,a sterilant solution may be provided pre-packed in a capsule which maybe placed in an adapted nebulizer so that the capsule is in contact withthe ultrasonic transducer of the nebulizer. In this case means areprovided for piercing the capsule so that it is able to release thesolution as a nebulant. In another embodiment the sterile solution maybe provided in a capsule having an integral ultrasonic transduceradapted to be energised via contacts extending through the capsule wallwhen the capsule is inserted in the nebulizer.

The nebulizer 6 need not be ultrasonic, and any other means for formingan aerosol could be used including sprays, jets, and other devices. Itis conceivable that peroxide could be pre-packed and stored as anaerosol in an aerosol container and could be admitted from the aerosolcontainer. It is also envisaged that cassettes incorporating anultrasonic transducer could be used to generate an aerosol in-situwithin the enclosed container which would be provided with electricalconnections to the exterior to provide for energisation and control.

The nebulizer 6 operates preferably at around 2.4 MHz to form an aerosolwith typically more than 90% of the droplets being between 1 and 10 μmin diameter, with the median size being around 3-5 μm in diameter(“micro particles”)

Although the present invention has been described with reference tonebulization by means of an ultrasonic nebulizer, it will be understoodthat other means for nebulization including sprays, jet nebulizers,piezoelectric nebulizers, and such like nebulant generating devices maybe employed. As described in our co-pending application(PCT/AU99/00505), smaller particles can be obtained by including asurfactant for example an alcohol, in the sterilant solution when usingultrasonic nebulization. It is not necessary for an ultrasonic nebulizerto be run continuously and in preferred embodiments of the invention thenebulizer is switched on and off cyclically, (or at irregular intervals)being run for example about 20 seconds per minute.

The aerosol or nebulant of microparticles is then propelled into thepre-concentrator 4 by means of a fan 8 upstream of the nebulizer 6. Themicroparticles formed by the nebulizer 6 are entrained in a gas streamwhich in the preferred embodiment is air. It is a significant advantageof preferred embodiments of the invention over prior art that they donot require a source of filtered sterile air. Instead the invention isable to draw non-sterile air from the sterilization chamber, andsterilize it while recirculating it in use. However, if preferred,aseptic filtered air could be employed. The gas stream is notnecessarily air, and could for example be an inert gas such as nitrogen,or argon; or could be oxygen or ozone.

In general terms, the pre-concentrator 4 works by exposing the aerosoldroplets to one face 10 of a semi permeable membrane 9 while an aircurrent moves across the other face 11 of this semi permeable membrane.This leads to preferential evaporation of the water from the aerosoldroplets, causing them to become more concentrated with respect tohydrogen peroxide. As a result of the preferential evaporation of water,the aerosol droplets inside the pre concentrator 4 become moreconcentrated with respect to hydrogen peroxide with the concentrationsapproaching 60% or upwards. Water continues to preferentially evaporatefrom the droplets until this maximum hydrogen peroxide concentration isachieved, after which peroxide and water evaporate in an equilibriumfixed proportion.

Once formed, the small highly concentrated droplets then make contactwith the article to be sterilised.

There are two possible preferred modes of operation of thepre-concentrator:

In the first operating mode, which is a batch-wise concentrationprocess, the pathway between the concentrator 4 and sterilizing chamber2 is shut and an aerosol of a solution of 35% hydrogen peroxide in waterwith droplet sizes between 1 and 10 μm is driven into thepre-concentrator chamber 4. The pre-concentrator chamber is thenisolated (by shutting both valves 5 and 12) and the aerosol in thepre-concentrator 4 is then concentrated. Concentration in thepre-concentrator takes place until the maximum concentration of peroxideis achieved, beyond which peroxide and water evaporate in an equilibriumfixed proportion. Once this maximum concentration is achieved, thepathway between the pre-concentrator and sterilizing chamber is openedby opening valve 5 and the concentrated nebulant is introduced into thesterilization chamber 2.

In the second alternative operating mode, which is a continuousconcentration process, the pathway between the pre-concentrator 4 andthe sterilization chamber 2 is left open. An aerosol of a solution of35% hydrogen peroxide in water with droplet sizes between 1 and 10 μmenters the pre-concentrator chamber 4 and passes continuously throughthe pre-concentrator with fan 8 propulsion. As the aerosol droplets passthrough the pre-concentrator 4, the water is preferentially removed.Residence time of the droplets in the pre-concentrator is such that themaximum possible concentration of peroxide is achieved by the time theyexit the pre-concentrator.

The nebulant may be introduced into the pre-concentrator 4 continuouslyor intermittently, for example, 2 secs on/18 secs off; or 5 secs on/15secs off; over a period of, for example, 2 minutes.

However, regardless of whether batch-wise mode a) or continuous mode b)is employed, or even should some combination of continuous or batch wisemodes be used, the aerosol droplets that exit the pre-concentrator 4 andenter the sterilization chamber 2 are at their maximum achievablehydrogen peroxide concentration.

As the concentration of hydrogen peroxide in the droplets increases, theproportion of hydrogen peroxide in the vapour in equilibrium with thedroplets increases.

Once the concentrated nebulant is introduced to the sterilizationchamber 2, it contacts the article to be sterilized 1 and acts upon thepathogens at the surface. The sterilizing chamber 2 may then be sealedfrom the pre-concentrator 4. Because the peroxide concentration is at amaximum, no further concentration of the peroxide solution takes placein the sterilizing chamber 2. Any vaporization in the sterilizationchamber takes place such that peroxide and water evaporate in anequilibrium fixed proportion. The concentrated biocide is then allowedto contact the article to be sterilized. The article to be sterilizedcan be stored in the sterilization chamber until needed. This alsopermits removal of residual hydrogen peroxide and water.

To expand on each of the steps, and shown in FIG. 4, the cycle commenceswith nebulization of 27-35% hydrogen peroxide into micro droplets insidea nebulization chamber 6 using an ultrasonic piezo ceramic transducerthat vibrates at 2.4 MHz. The transducer may function continuously oraccording to an appropriate duty cycle such that nebulization isintermittent. The nebulant mist has micro-droplets which have the samecomposition as the bulk solution from which they were derived.

Once produced, the nebulant mist is transported by a blower fan 8 intothe membrane concentrator system 4 where it is concentrated by means ofevaporation into sub-micron particles or nano-nebulant.

The membrane concentrator 4 is preferably a multi-layered device wherenebulant flows over membrane layers which have an alternate airflow onthe other side. Selective removal of a proportion of the water vaporfrom the nebulant occurs in the membrane concentrator due to thedifferential partial pressures of water and hydrogen peroxide. Theconcentrator may be electrically heated if required to provide thedesired effect. Not only do the droplets become more concentrated(˜60-70%), because of the loss of solvent (water) they become smaller.The smaller droplets also increase surface area/volume and so becomemore stable. The net result is an ultra-fine, stable and concentratedmist or nano-nebulant. At the exit point of the concentrator the mist is“terminally” concentrated such that no further concentration of hydrogenperoxide occurs in the sterilization chamber.

In one simple embodiment, seen in FIG. 2, the membrane concentrator is amodular, stackable assembly consisting of 4 main components—flow layer,end plate, tie-rod and membrane sheet. FIG. 5 shows a preferred stack ofconcentrator modules.

The flow layers 10 and 11 are defined by thin, square or rectangularplates 12 with a large open area inside and four slots (galleries)running parallel to the outer edges, two of which are connected to theinner space via slots. The orientation of the flow layers (when usingsquare sections), determines the number of layers which are common toany particular gallery, hence two distinct flow lines may operate an onesingle assembly through the method of assembly.

The end plates 13 allow connection of external tubing or devices to themembrane assembly and each end plate has two connection points whichcorrespond to two gallery slots. The slots on these end plates form amanifold which directs flow up one particular gallery per connection andthe connections are offset 90 degrees from one another to ensure theyaccess different galleries.

When five flow layers, for example are stacked atop one another withalternate orientations i.e. 90 degrees to each other, and separated bysheets of membrane material, they form two groups of flow layers, onehaving two flow layers 15 and the other having three separate flowlayers 16 within the block. These flow layers are assigned to eithernano-nebulant (15 in the present case) or crossflow/counterflow (16 inthe present case) connections and through regulation of their flowrates, controlled diffusion is possible.

The tie-rods are used to compress the layers between the end plates andcreate a vapor seal, although any design which allows the blocks to fittogether in suitable sealed arrangement may be used. The membranematerial 9 also acts as a gasket between the layers.

Whilst the vapour pressure of hydrogen peroxide at ambient temperaturesis negligible, and water preferentially evaporates in the membraneconcentrator, as a precaution against any hydrogen peroxide flow exitingthe system, the counter flow is taken directly into the catalyticdestructor module where it is safely treated.

The semi-permeable membrane 9 in the present example is preferably madeof KIMGUARD™, a three layer non linting laminate fabric usingpolypropylene and having an inner layer which is hydrophobic andresistant to bacterial penetration. The two outer layers provideabrasion resistance and strength. As a multi layered fabric it has noactual pore size, but the fabric is permeable by virtue of microscopicchannels which provide a tortuous path limiting passage of particles tothose of less than 0.2 micron, ie it is impermeable to micro-organismsbelow 0.2 microns. This fabric allows water and hydrogen peroxidevapours to permeate through the channels of the fabric. The channels donot permit passage of bacteria into the chamber and do not permitnebulant to pass out. Kimguard has a hydrostatic repellency of 3.8 kPa(measure of hydrophobicity) and a cross dimensional tensile load of 70Newtons and a machine directional tensile load of 130 Newtons.

The semi permeable membrane 9 may be any other suitable semi permeablemembrane which facilitates the removal of water while being impermeableby micro-organisms and nebulant particles. Other fabrics and membraneswhich are permeable by water vapour and hydrogen peroxide vapours andimpenetrable by bacteria may be used, for example TYVEK™ and SPUNGUARD™(However, KIMGUARD™ has been found to be 2-3 times more permeable tohydrogen peroxide vapour than TYVEK™ under the conditions in which it isused here. As will be discussed hereinafter other semi permeablemembrane materials such as NAFION™ (which is hydrophilic) and the likemay also be employed.

NAFION™ is a copolymer of tetrafluoroethylene and perfluoro 3, 6,dioxa-4-methyl-octene-sulphonic acid. Such materials are hydrophilic andhave a very high water of hydration. NAFION™ is able to absorb 22% byweight of water. In this variation the absorption proceeds as a firstorder kinetic reaction. Water molecules pass through the membrane andthen evaporate into the surrounding air until equilibrium with theexternal humidity is reached in a continuous process calledpervaporation. An exterior current flow of air over the external side ofthe membrane provides rapid removal of the moisture from the outsidesurface and speeds the pervaporation process. Unlike simple permeationwherein the molecules merely diffuse through the open pores, inpervaporation the membrane is active in selectively drawing moleculesfrom one side of the membrane to the other, and may do so atdifferential rates for differing types of chemical molecule.

In the embodiments described above the sterilizing agent is a solutionof hydrogen peroxide as a 35 wt % solution in water which acted as thesolvent. Water is the preferred solvent for use with peroxide. Waterboils at 100° C. while hydrogen peroxide boils at above 151° C. atatmospheric pressure. Hydrogen peroxide boils at 151.4° C. at 760 mm.FIG. 1 taken from U.S. Pat. No. 4,797,255 shows (curve A) how theboiling point at atmospheric pressure of a water/peroxide mixturechanges with concentration and (curve B) how the gas compositionchanges. As is shown, pure water boils at 100° C. at atmosphericpressure. It is evident from FIG. 1 that the concentration of hydrogenperoxide in the vapour at below 100° C. is negligible at atmosphericpressure. The solvent could for example be an aqueous or non-aqueousalcohol chosen in combination with the sterilizing agent to be used. Theaddition to water of ethyl alcohol results in an azeotropic mixturewhich lowers the boiling point of the solvent and this enables the waterto be “flashed” off at lower temperatures than would otherwise bepossible. The addition of other azeotropic agents would be equallybeneficial. The use of azeotropes to facilitate the removal of solventfrom the nebulant solution particles is within the scope of theinvention. It is envisaged that for some biocides non-aqueous solventsor a combination of suitable solvents could be employed.

In the case of hydrogen peroxide, as the water flashes off, theconcentration of the sterilizing agent increases. If a 35% peroxidesolution is used in the invention the micro-nebulant after the heatingand water vapour removal steps will have a concentration of for example60 to 80%. This has the advantage that the starting material can behandled relatively safely, that concentration occurs during the processand that thereafter there is no further need to handle the peroxide.Also, the average particle size is greatly reduced, the micro nebulantparticles in preferred embodiments having a mean diameter of less than 1micron, more preferably less than 0.1 micron. The small particle sizeresults in a very stable suspension with negligible settling out,provides a significant increase in the liquid/gas interfacial area, andin very high concentrations of liquid sterilant per litre of nebulant.The inventors believe that there may be a higher concentration ofperoxide molecules at the gas/liquid interface in these nano particlesthan occurs in micro particles. Solutions of a lower or greaterconcentration than 35% can be used as a starting material and excellentresults have been obtained with hydrogen peroxide solutions of 1% or 3%as well as with solutions of 40%, but the time taken to achieve asatisfactory result with mated or occluded surfaces was less thanoptimum with peroxide concentrations below 30%, and handling issuesresult in a preference for concentrations of below 35%. While preferredembodiments described have employed aqueous solutions of hydrogenperoxide as the sterilizing agent, solutions of other peroxides andperoxy compounds can be employed as well as solutions of peroxycomplexes (including non water soluble complexes in organic solvents).Sterilizing agents other than peroxides may also be used in theinvention, including without limitation halo compounds, phenoliccompounds, halogen phenolic compounds and other known biocides, withappropriate choice of solvent.

Whilst concentrations of peroxide in droplets produced from 30-35%peroxide solution typically approach 60% or upwards, it is not alwaysnecessary that such a high peroxide concentration is achieved. Forexample, in other preferred embodiments, a starting solution which has aconcentration of 10 to 15% peroxide is nebulised and concentrated toaround 45 to 60% peroxide. Any starting concentration of peroxide can beused, and concentrated to any level up to the theoretical maximumachievable under the prevailing conditions of relative humidity andtemperature. Generally, in practical terms, a peroxide concentration of10-15% to 30-35% is employed as the starting solution, which isconcentrated up to 45-60% or above in the nebulant.

In an example in which the article to be disinfected is the part of anultrasonic probe 20, for example a probe of a type insertable into abody cavity for diagnostic purposes, the part of the probe 20 to betreated is enclosed in a chamber 2 (as exemplified in FIG. 9). In thiscase the chamber is a specially shaped chamber designed so that thewhole article need not be in the chamber, only that part of the probewhich is to be treated being enclosed. The probe can be suspended insidethe chamber by means of a seal around the gland where the power cordenters the probe.

The nano-nebulant is then transported into chamber 2 where it is appliedto a target surface. The ultrasound device may be inserted into thechamber via any of the panels on the device. One possible entrance isfrom the top via a screw top lid into which the cord of the device isclamped and held in place on insertion into the chamber. Passage of thenano-nebulant from the concentrator to the chamber is regulated by acheck valve 5. Check valves 5 and 12 can control whether the deviceoperates batchwise, continuously or by some combination of both.

If the device operates batchwise, the valve 5 is opened at theappropriate time after the concentration has occurred.

If the device is operated continuously, the valve remains open, with theflow rates and residence times of the nebulant calibrated beforehand tobe at a maximum when exiting the chamber.

Typically, the chamber 2 is constructed of a heat conductive metal suchas stainless steel or aluminium. Various coating may be applied to theinterior of the chamber such as Teflon to reduce the risk of peroxidebreakdown. The disinfection chamber is electrically heated using heatertrace wire applied to the conductive metal surface. Alternatively, or inaddition, heated air can be blown into chamber. Chamber atmosphere tosupply the blower is made-up from another chamber connection which isplaced on the opposite side of the chamber to the inlet. The chamberitself is isolated from the generation and recirculation circuit bymeans of valves which engage once the nano-nebulant cycle is complete(about 1-1.5 min). This isolation from the adjoining circuit is called“suspended time” or more commonly “hold” time.

The surface of the object 1 to be treated with the nebulant is exposedto the nano-nebulant particles for a time sufficient to sterilize thesurface. Surprisingly, it has been found that the resultingnano-nebulant is not only more rapidly effective than prior artaerosols, but also is highly effective at penetrating mated surfaces,and treating occluded surfaces which are not directly exposed. While itis not clear why this is so, it may be that a very high density ofnano-nebulant (for example 2.0 mg/l or greater at 40% RH) is distributedthroughout the volume of the sterilization chamber while at the sametime there is little or no actual condensation on the surface. Thenano-nebulant particles have a far greater surface area at thegas/liquids interface than the original micro nebulant particles and aresignificantly smaller in diameter, and consequently remain suspended formuch longer periods. Without wishing to be bound by theory, the presentapplicants believe that the nano-particles impinge on the surface at agreater frequency than the prior art micro particles, and have a longerresidence time on the surface than vapour molecules. In comparison withprior art aerosol processes, surfaces treated by the invention may berapidly dried and are relatively uncontaminated by residual peroxide.When treating a lumen, it is preferred that the lumen be connected toreceive a flow of the nebulant through the lumen. Desirably, theexternal and mated surfaces are also exposed to the nebulant in thechamber or cassette.

The chamber 2 may be formed fully of a semi permeable membrane or fabricor may have a wall of which at least a part is a semi permeable membraneor fabric may be of any suitable shape and design having regard to therequirements of the process herein described and can be sealed in anymanner impenetrable by micro organisms. Other semi permeable membranesor fabrics can be selected based on the teaching herein provided. Thecontainer may be permanently connected to the nebulizer circuit or maybe able to be connected and disconnected by a tube and spigotconnection, by suitable connectors or other means.

Once the suspended time is complete (approx 1-2 mins), the system movesinto catalytic destruction mode or simply “empty”. It is in this cyclethat a suction fan engages which pilots (opens under pressure) a checkvalve that connects to the chamber while another valve allows fresh airto enter the chamber at a controlled rate. This cycle moves thenano-nebulant into the catalytic destructor module where a catalyst isused to convert the hydrogen peroxide into harmless water vapor andoxygen. The catalytic destructor module is composed of metal oxide bakedceramic honeycomb layers sandwiching similarly treated ceramic beadspackaged in a suitable container. The amount of catalyst is proportionalto the amount of peroxide extracted from the chamber as well as the flowrate from the chamber. Completion of this cycle takes approximately 1minute and upon completion, the chamber may be accessed to retrieve thedisinfected target device. In this configuration the total cycle timefor high level disinfection approximates 5 minutes or less. It isunderstood that the time to achieve sterilization is more onerous andmay take significantly longer.

In some preferred embodiments, the droplet density in the aerosolpassing from the preconcentrator to the sterilization chamber may bemeasured by passing an infra red beam across the connecting conduit to adetector and measuring the beam attenuation. This varies with aerosoldroplet density and gives a measure of the quantity of peroxideliquid/unit time entering the sterilization chamber. The infra red ispreferably of a frequency which is not absorbed by peroxide per se andthus does not register peroxide vapour if any. A knowledge of theaerosol density, temperature and residence time allows certification ofthe result if desired.

The preconcentrator can be operated in such a manner that it alwaysoutputs nebulant comprising peroxide at a predetermined theoreticalmaximum concentration, thereby avoiding the need to determine theconcentration of peroxide at any point of the sterilizing process.

EXAMPLES

FIG. 8 shows the resulting concentration of peroxide following the useof the membrane concentrator of the present invention. FIG. 8 compares %relative humidity (RH) and peroxide (H₂O₂) levels (ppm) measured withina 3 liter chamber with a flow rate of aerosol at 9I/min into themembrane concentrator described above, or bypassing it entirely. Thestarting concentration of peroxide was 30%. The membrane used in thiscase was KIMGUARD although similar profiles are obtained with NAFION andTYVEK.

Bypassing the membrane concentrator/module (FIG. 8 a) reveals 46%relative humidity and a peroxide level of about 980 ppm.

However, when the membrane concentrator is employed, it can be seen(FIG. 8 b) that the corresponding concentration of peroxide is over2100, and the relative humidity dropped to 28%. In effect, the use ofthe pre-concentrator of the present invention has removed a large amountof the water, leading to more than doubling of the peroxideconcentration

Tables 1, 2 and 3 below indicate that increasing the counter flowresults in increased the concentration of peroxide in the 3 L chamberover a 5 minute period with NAFION showing the greatest effect.

TABLE 1 Influence of counter-flow velocity in NAFION membrane module onratio between hydrogen peroxide and water by weight in disinfectionchamber at 50° C. Condition of nebulant/ Velocity of counter-flownano-nebulant supply L/min Ratio H₂O₂/H₂O Bypasses concentrator N/A0.033 Through concentrator 0.0 0.061 4.5 0.108 7.5 0.118 9.0 0.088 12.00.102

TABLE 2 Influence of counter-flow velocity in TYVEK membrane module onratio between hydrogen peroxide and water by weight in disinfectionchamber at 50° C. Condition of nebulant/ Velocity of counter-flownano-nebulant supply L/min Ratio H₂O₂/H₂O Bypasses concentrator N/A0.033 Through concentrator 0.0 0.046 4.5 0.083 7.5 0.082 9.0 0.080 12.00.58

TABLE 3 Influence of counter-flow velocity in KIMGUARD membrane moduleon ratio between hydrogen peroxide and water by weight in disinfectionchamber at 50° C. Condition of nebulant/ Velocity of counter-flownano-nebulant supply L/min Ratio H₂O₂/H₂O Bypasses concentrator N/A0.053 Through concentrator 0.0 0.063 4.5 0.112 7.5 0.149 9.0 0.125 12.00.109

Table 4 below indicates the effect of the nano-nebulant process usingthe membrane concentrator on carriers inoculated with 5×10⁶ cfu B.stearothermophilus/carrier with 400 ppm hard water and 5% horse serum.Flow rate of aerosol was 9l/min, counter flow was 9l/min, temperature inthe chamber was 50° C. and starting concentration of peroxide was 30%.Peroxide delivered was 0.11 g/l.

TABLE 4 Relationship of time to Spore Reduction on different surfaceconditions. Stainless steel Porcelain Stainless steel mated Time ofpenicylinders washers washers 85 mm² cfu/ exposure (log reduction) (logreduction) (log reduction) carrier (min) n = 50 n = 10 n = 3 5 × 10⁶ 12.6 5.9 2.1 5 × 10⁶ 2 5.8 >6 4.3 5 × 10⁶ 5 >6 >6 5.2 5 × 10⁶ 10 >6 >6 >6

The following are illustrative of the types of particle sizes that canbe obtained by the pre-concentrators of the present invention. Table 5shows the particle size distribution of a nebulant from an ultrasonicnebulizer fed with 30% hydrogen peroxide solution at varioustemperatures. These would represent the input particle sizes for thepre-concentrators of the present invention.

TABLE 5 Heater's outlet T 10% below 50% below 90% below ° C. (particlesize, μm) (particle size, μm) (particle size, μm) 25 2.84 5.5 9.48 550.95 1.36 2.0 60 0.58 0.86 1.36

Table 6 shows the particle size data at 25° C. of the nebulant when aNAFION membrane was used with various airflow rates on the exteriorside.

TABLE 6 Counter 10% below 50% below 90% below flow m/s (particle size,μm) (particle size, μm) (particle size, μm) 0 2.29 4.61 8.58 3.2 2.333.99 6.36 7.5 2.0 2.9 3.96

Table 7 shows the particle size data at 25° C. of the nebulant when aKIMGUARD membrane was used at various flow air flow rates on theexterior side.

TABLE 7 Counter 10% below 50% below 90% below flow m/s (particle size,μm) (particle size, μm) (particle size, μm) 0 2.29 4.61 8.58 3.2 2.314.17 7.2 7.5 2.57 4.2 6.51

Particle size can be seen to have dropped by about half in the case ofNafion (corresponding to a droplet volume reduction to about 30%original size) and about one third in the case of Kimguard(corresponding to a droplet volume reduction to about 13% originalsize).

Although the invention has been herein described with reference tohydrogen peroxide as the sterilizing agent, the invention could useother peroxides, peroxy-compounds, or complexes of either. Other classesof biocide could be used including without limitation halogenatedbiocides, phenolic biocides and quaternary compound biocides and it maybe advantageous to use solvents other than water. Likewise, although theinvention has been herein exemplified primarily with reference tostarting solutions having 35% peroxide, other starting concentrationscan be employed, although concentrations between about 20% and 35% arepreferred.

The principles herein taught could be applied to concentrate theperoxide in such vapour processes by permeation or pervaporation througha membrane, without the need for pressure reduction. However thebenefits (described in our co-pending application) of utilizing theaerosol of the invention would be lost as a sterilant would be lost.

1-11. (canceled)
 12. A method for concentrating a nebulant to produce aconcentrated nebulant and wherein the concentrated nebulant is used todisinfect and/or sterilize an article, the method comprising: (1)providing a flow of a nebulant consisting of a biocide in a solvent andhaving a first biocide:solvent ratio to a first side of a gas permeablemembrane; and (2) providing a counter-flow of a gas to a second side ofthe gas-permeable membrane whereby to increase said firstbiocide:solvent ratio on the first side to a second biocide:solventratio greater than the first biocide:solvent ratio.
 13. (canceled)
 14. Amethod according to claim 12 wherein the solvent is water.
 15. A methodaccording to claim 12 wherein the biocide is selected from the groupconsisting of hydrogen peroxide or a peroxy compound.
 16. A methodaccording to claim 12 wherein the first biocide to solvent ratio isbelow 35_wt %.
 17. A method according to claim 12 wherein the secondbiocide:solvent ratio is above 60_wt %.
 18. A method according to claim12 wherein the counter-flow of gas is provided at a rate and for a timesuch that the second biocide:solvent ratio reaches an equilibrium ratiobeyond which it will not increase.
 19. A method according to claim 12wherein the gas is air or humidity conditioned air.
 20. A methodaccording to claim 12 wherein the semi permeable fabric or membrane is awoven, or non-woven fabric, or a sheet or film or a combination thereofand of a single layer or multilayer construction.
 21. A method accordingto claim 12 wherein the semi permeable membrane is hydrophobic.
 22. Amethod according to claim 12 wherein the semi permeable membrane isselected to ensure that nebulant particles at the first biocide:solventratio are initially unable to permeate there through.
 23. A methodaccording to claim 12 wherein the membrane is suitable forpervaporation.
 24. A method according to claim 12 wherein the nebulantis an aqueous peroxide solution having an initial concentration of from6 wt %-35 wt % of peroxide.
 25. A method according to claim 24 whereinthe solution is nebulized in an ultrasonic nebulizer operated at greaterthan 2.0 MHz and which generates an aerosol in which particles having asize range distribution of about 1-10 microns are suspended in an airstream.
 26. A method according to claim 12 wherein the semi permeablemembrane is selected to remove one or more vapours by a process ofpervaporation. 27-33. (canceled)
 34. A method according to claim 12wherein the nebulant at the first biocide:solvent ratio has 90% ofparticles in the 3-5 μm range.
 35. A method according to claim 12wherein the nebulant at the second biocide:solvent ratio has an averageparticle diameter of less than 1.0 micron.